1. Field of the Invention
The invention relates to a wireless communication technique and more particularly to an in-phase to quadrature (I/Q) mismatch correction technique utilized by a receiver.
2. Description of the Related Art
FIG. 1 shows a conventional receiver 100. Receiver 100 is designed based on the Weaver structure, including mainly a radio frequency module 110 and an intermediate frequency module 120. The radio frequency (RF) module 110 receives a radio frequency signal RF and down converts the radio frequency signal RF into an intermediate frequency signal IF, and the intermediate frequency module 120 further down converts the intermediate frequency signal IF into a baseband signal LF. The signal path is divided into an in phase path (I-path) and a quadrature phase path (Q-path). As an example, in the radio frequency module 110, a first Phase Locked Loop (PLL) PLL1 provides oscillation signals LO1I and LO1Q oscillating at a first oscillation frequency LO1. The first mixer 102a and the second mixer 102b respectively mix the oscillation signals LO1I and LO1Q with the radio frequency signal RF. The obtained mixed signals are passed to the first low pass filter 104a and the second low pass filter 104b to generate the intermediate frequency signal IF, which comprises the in phase intermediate frequency signal and the quadrature phase intermediate frequency signal Q1. In the intermediate frequency module 120, the second phase locked loop PLL2 provides oscillation signals LO2I and LO2Q oscillating at a second oscillation frequency LO2 to the third mixer 106a and the fourth mixer 106b. The in phase and quadrature phase intermediate frequency signals I1 and Q1 are respectively mixed by the third mixer 106a and the fourth mixer 106b. The obtained in phase and quadrature phase baseband signals I2 and Q2 are added together by the adder 108 to obtain the output baseband signal LF.
The main purpose of the receiver 100 is to filter out the image (so-called image rejection) generated in the down conversion process. Ideally, when the in phase path circuit and the quadrature phase path circuit are perfectly matched, the image can be perfectly rejected. Generally, when the efficiency index, so-called Image Rejection Ratio (IRR), is higher, the image rejection performance is better. However, when implementing the circuit in practice, due to several imperfect factors, amplitude mismatch, gain mismatch and phase mismatch usually exist between the in phase path and the quadrature phase path, which is called I/Q mismatch. The I/Q mismatch seriously degrades the performance of image rejection. The gain mismatch of a circuit normally ranges between +−10%, and the phase mismatch ranges between about +−5%. In order to improve the impairment caused by I/Q mismatch, an additional I/Q calibration circuit is adopted in the conventional design to detect the mismatch between the in phase path and the quadrature phase path and generate a corresponding compensation value. As an example, most of the detection and calibration methods use an adaptive circuit structure to detect and compensate the mismatch. The advantage of the adaptive circuit structure is that design thereof is flexible and it is able to eliminate the factors influencing the I/Q mismatch. However, the adaptive circuit structure normally requires a lot of time for training, for satisfactory compensation results. In addition, in order to detect mismatch, an additional test signal #CAL is needed. Because the signal to noise ratio (SNR) requirement of the test signal #CAL is usually high, additional costs are required to improve the quality of the SNR.